Evolution of structure and magnetic properties for BaFe11.9Al0.1O19 hexaferrite in a wide temperature range

Author’s Accepted Manuscript EVOLUTION OF STRUCTURE AND MAGNETIC PROPERTIES FOR BaFe11.9Al0.1O19 HEXAFERRITE IN A WIDE TEMPERATURE RANGE A.V. Trukhanov, S.V. Trukhanov, L.V. Panina, V.G. Kostishyn, I.S. Kazakevich, V.A. Turchenko, M.M. Salem, A.M. Balagurov

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To appear in: Journal of Magnetism and Magnetic Materials Received date: 15 July 2015 Revised date: 21 October 2016 Accepted date: 27 October 2016 Cite this article as: A.V. Trukhanov, S.V. Trukhanov, L.V. Panina, V.G. Kostishyn, I.S. Kazakevich, V.A. Turchenko, M.M. Salem and A.M. Balagurov, EVOLUTION OF STRUCTURE AND MAGNETIC PROPERTIES FOR BaFe11.9Al0.1O19 HEXAFERRITE IN A WIDE TEMPERATURE RANGE, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.10.140 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

EVOLUTION OF STRUCTURE AND MAGNETIC PROPERTIES FOR BaFe11.9Al0.1O19 HEXAFERRITE IN A WIDE TEMPERATURE RANGE A.V.Trukhanov1,2*, S.V.Trukhanov1,2, L.V.Panina1, V.G.Kostishyn1, I.S.Kazakevich2, V.A.Turchenko3, 4, M.M.Salem1, A.M.Balagurov3 1

National University of Science and Technology MISiS, 119049, Moscow, Leninsky Prospekt, 4, Russia

2

SSPA “Scientific and practical materials research centre of NAS of Belarus”, 220072 Minsk, P. Brovki str., 19, Belorussia

3

Joint Institute for Nuclear Research, 141980 Dubna, Joliot-Curie str., 6, Russia

4

Donetsk Institute of Physics and Technology named after A.A. Galkin of the NAS of Ukraine, 83114, Donetsk, 72 R.Luxemburg Str., Ukraine

*

Corresponding author. [email protected]

Abstract M-type BaFe11.9Al0.1O19 hexaferrite was successfully synthesized by solid state reactions. Precision investigations of crystal and magnetic structures of BaFe11.9Al0.1O19 powder by neutron diffraction in the temperature range 4.2 – 730 К have been performed. Magnetic and electrical properties investigations were carried out in the wide temperature range. Neutron powder diffraction data were successfully refined in approximation for both space groups (SG): centrosymmetric #194 (standard non-polar phase) and non-centrosymmetric #186 (polar phase). It has been shown that at low temperatures (below room temperature) better fitting results (value χ2) were for the polar phase (SG: #186) or for the two phases coexistence (SG: #186 and SG: #194). At high temperatures (400-730 K) better fitting results were for SG: #194. It was established coexistence of the dual ferroic properties (specific magnetization and spontaneous polarization) at room temperature. Strong correlation between magnetic and electrical subsystems was demonstrated (magnetoelectrical effect). Temperature dependences of the spontaneous polarization, specific magnetization and magnetoelectrical effect were investigated. PACS: 61.05.fm, 61.50.Nw, 75.10.-b Keywords: barium hexaferrites, multiferroic, magnetization, polarization, magnetoelectrical effect, pyroelectricity, neutron diffraction. 1. Introduction Ferroelectrics are materials that display an electric polarization in zero applied electric field in which the direction of the electric polarization is reversed by the electric field. Ferroelectric order results from the formation of an electric moment within the unit cell of the crystal and can only exist in a material with broken inversion symmetry. Ferromagnets are magnetically ordered 1

materials in which all the magnetic moments are aligned in the same direction in zero applied magnetic field and in which time-reversal symmetry is broken. Magnetoelectric (ME) multiferroics are the materials that exhibit not only ferroelectric and magnetic order simultaneously but also display coupling between these properties. [Although the term multiferroics was originally coined for materials in which two or all three ferroic orders (ferroelectric, ferromagnetic, and ferroelastic) coexist in the same phase [1], it refers only to ME multiferroics in this review.] In the past decade, multiferroics have been attracting renewed interest and have been extensively studied in terms of both fundamental and technological points of view [2–5]. In multiferroics, the ME effect—the generation of magnetization by an electric field and generation of electric polarization by a magnetic field—can be anticipated. The ME effect was first postulated at the end of the nineteenth century by Pierre Curie. Since the theoretical prediction for the ME effect in Cr2O3 by Dzyaloshinskii in 1960, the effect has attracted continuous interest for more than a half century, and has generated renewed attention related to the search for multiferroics in the past decade. Multiferroics are naturally classified by the physical origin of their inversion symmetry breaking [6, 7] [e.g., 6s2 lone pair [8], geometric structural transition [9], charge ordering [10–12], and magnetic ordering [13]]. Among these various multiferroics, extensive studies of ferroelectrics originating from magnetic orders, i.e., magnetically induced ferroelectrics in which the inversion symmetry breaking and resultant ferroelectricity are induced by complex magnetic orders, were triggered almost a decade ago by the discovery of multiferroic nature in a perovskite-type rare-earth manganite TbMnO3 [14]. The magnetically induced ferroelectrics often show giant ME effects, remarkable changes in electric polarization in response to a magnetic field, because the origin of their ferroelectricity is driven by magnetism that sensitively responds to an applied magnetic field. Thus, it is expected that the magnetically induced ferroelectrics provide new types of device applications by using the ME effect, such as memory devices in which magnetic and/or ferroelectric domains are controlled by an electric and/or magnetic field. Though several new magnetically induced ferroelectrics have been reported in the past decade, so far there has been no practical application employing the ME effect of the magnetically induced ferroelectrics. This is partly because none of the existing magnetically induced ferroelectrics have combined large and robust electric and magnetic polarizations at room temperature until quite recently. Until recently, barium ferrite with the hexagonal structure of magnetoplumbite (М-type) BaFe12O19 was widely used only as permanent magnets [15] and in high-density information magnetic storage devices with a perpendicular magnetization [16]. However, the M-type barium hexaferrite has recently found a new, third application as a multiferroic, i.e., a material exhibiting a significant coupling between magnetic and dielectric properties [17, 18]. Such materials will be widely applied in spintronics, which is a new field of microelectronics [19]. Recent studies have been shown [20] that the BaFe12O19 is a perspective Pb-free multiferroic material. The large spontaneous polarization was observed for the BaFe12O19 ceramics at room temperature [20]. The maximum remanent polarization was estimated approximately 11.8 µC/cm2. Authors assumed that the main reason of polarization in collinear centrosymmetric structure is in FeO6 octahedron distortion (exchange-striction mechanism). But there isn’t any structural information to prove their point of view. The magnetic field which induced electric polarization has been also observed in the doped BaFe12-x-δScxMδO19 (δ=0.05) at 10 K [21] and in the BaScxFe12−xO19 and SrScxFe12−xO19 (x = 1.3–1.7) single crystals of M-type hexaferrites [22]. And the main mechanism of dual ferroic properties formation in this case is noncollinear magnetic structure formation. The aim of this work is the search for new intense room temperature multiferroics and determination of the mechanism of spontaneous polarization in M-type barium hexaferrites replaced with diamagnetic aluminum cations BaFe12–xAlxO19 (x =0.1). We consider a sample with x = 0.1. We have recently studied the structure and magnetic properties of aluminum-substituted barium hexaferrites BaFe12–xAlxO19 (x =0.1-1.2) at room temperature [22]. And now our purpose is to

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investigate temperature dependences of magnetic and electrical properties and discuss the mechanism of dual ferroic properties formation in collinear magnetic structure. 2. Experiment The investigated BaFe11.9Al0.1O19 sample has been obtained from high purity Fe2O3 and Al2O3 oxides and carbonate BaCO3 by solid state reactions in accordance with [22]. The precision study of the crystal and magnetic structures in the temperature range of 4.2–730 K was performed by the power neutron diffraction method on a high-resolution Fourier diffractometer. This diffractometer is a time-of-flight diffractometer at the IBR-2M pulsed reactor (Dubna, Russia) with a relatively large (~21.131 m) path length from moderator to a detector. It has an exceptionally high resolution (Δd/d ≈ 0.001), which hardly depends on the interplanar distance in a wide interval of dhkl. Highresolution neutron diffraction patterns were recorded by detectors located at the average scattering angles +152° in the interplanar distance interval from 0.6 to 3.6 A. The experimental time-of-flight neutron diffraction patterns were analyzed by the full-profile Rietveld method with the MRIA and FullProf suites with the use of incorporated tables for coherent scattering lengths and magnetic form factors. The resolution of the high-resolution Fourier diffractometer was determined in a special experiment by the Al2O3 reference. The specific magnetization was studied by VSM with the liquid-helium free high-field measurement system (Cryogenic Ltd, London, United Kingdom) at temperatures 4-730 K in fields up to 14 T. Magnetic measurements were performed on polycrystalline sample with the average dimensions of 2 × 3 × 5 mm. The spontaneous magnetization was determined by the linear extrapolation of the field dependence to zero field. The high-voltage electric polarization was measured at a temperature of 300 K and electrical field up to 100 kV/m with the use of an instrument described in detail in [23]. We used the configuration of the electric field in the form of bipolar pulses (sawtooth bipolar voltage) supplied to a measured capacitor. To measure the electric polarization, electrodes based on silver paste were used. An electric response was detected with a high-ohmic operational amplifier data from which were guided through an analog-to-digital converter to a PC. The samples were shortened before the measurements. Magnetoelectrical effect was determined from magnetic field dependences of specific magnetization in external electrical field (60 kV/m) and in zero electrical field. The electric field was perpendicular to the magnetic field E ⊥ B. Magnetoelectrical coefficient was calculated as: Kme = MS(0) - MS(E)/MS(0)*100 %

(1)

MS(0) – specific magnetization in zero external electrical field; MS(E) – specific magnetization in external electrical field 60 kV/m. 3. Results and discussion 3.1. Features of crystal structure Although the enormous amount of research has been devoted to hexaferrites, there are still a number of problems to be addressed and eventually solved. The problem involved is the clear understanding of the crystal structure of hexaferrites that is often an invaluable initial step in understanding synthesis-structure-property relationships. In spite of a huge number of structural studies spanning many years [24-26] (and references therein), the number of precise structure refinements of hexaferrites appearing in the literature is surprisingly small [27, 28]. Therefore, the literature data contain certain differences in the values of the main structural parameters (a, c) of hexaferrites [29]. Thus, in our opinion, a lot of published experimentally determined crystal parameters of hexaferrites can be considered as poorly characterized and may be mistaken.

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According to neutron data the investigated polycrystalline sample possesses a hexagonal structure with space group #194 (P63/mmc) with two molecules in the unit cell (Z = 2). Precise determination of the lattice parameters and atomic structure refinement were performed by neutron diffraction in a wide temperature range (from 4.2 K to 730 K) on the high-resolution Fourier diffractometer. The powder neutron diffraction patterns for the BaFe11.9Al0.1O19 at 4 K and 730 K are presented on Fig. 1. An impurity hematite phase Fe2O3 is also present with space group (R-3c), with six molecules in the unit cell (Z = 6), similarly to [22]. The weight percent of secondary Fe2O3 phase is 0.97 %. This is the result of slight dissolution of the BaO*Fe2O3 (tetragonal lattice) in the M-type hexaferrite. The impurity phase formation fact can be eliminated by introducing the synthesis of a small amount (0.4 mol%.) BaO. The high resolution of diffractometer and, correspondingly, the large number of well-separated peaks provided the good convergence of the minimization process. The calculation took into account the thermal vibrations of atoms - in the isotropic approximation and the magnetic hexaferrite structure - in the collinear approximation. The Rwp (weighted profile R-value), Rexp (expected R-value), RB (Bragg R-factor), RMag (magnetic R-factor) and χ2 (goodnessof-fit quality factor) parameters obtained after refinement for SG #194 are presented in Table 1. The low values of fitting parameters suggest that the studied sample is of better quality and refinements of neutron data are effective. According to our calculations, the Al3+ cations have equal probability for location in any position available for filling. In our previous paper we demonstrated that unit cell volume as well as lattice parameters (a and c) decrees with decreasing temperature [22]. The high anisotropy of crystal structure leads to difference of the coefficient of linear thermal expansion for a (αa~ 9.32*10-6 [1/K]) and с (αc~ 1.5*10-5 [1/K]) axis [22]. In low temperature range from 150 to 4.2 K the Invar effect (zero thermal expansion coefficients) is observed for all compositions. In this range decreasing temperature leads to low decrease of cell volume and с axis, whereas a axis increases. But it is a more interesting fact that, as a rule, all researchers who deal with M-type hexaferrites describe crystal structure of the samples by SG #194 (P63/mmc – Fig. 2a). This SG is centrosymmetric structure or “non-polar” phase. So, theoretically it can’t be in centrosymmetric SG non-zero dipole moment (electrical polarisation). But in practice a lot of researchers demonstrate coexistence of magnetization and polarization [14, 18, 21, 30-35] in M-type hexaferrites. But one of them reported that polarization is concerned with deviation of magnetic structure (formation noncollinear structures like helicoidally, conical or spiral magnetic structures) and other researchers reported that polarization is the evidence of crystal structure distortion (non-centrosymmetrical shifts of iron cation in some oxygen coordination). But if we deal with some distortion or deviation in centrosymmetrical structure (changing in crystal or magnetic structures) it must lead to phase transition from “non-polar” phase (centrosymmetrical structure) to “polar” phase (noncentrosymmetrical structure). And crystal structure can’t be described by SG #194 – this is a centrosymmetrical structure. But all researchers described crystal structure in the frame of SG #194 for more than 60 years. Our careful analysis of the BaFe11.9Al0.1O19 structure on atomic level suggests a perovskitelike crystal structure with one distorted FeO6 oxygen octahedron in hexagonal structure – Fig. 3 [35]. Each hexagonal model has one FeO6 oxygen octahedron in a sub-unit cell. In a normal octahedron, Fe cation is located at the centre of an octahedron of oxygen anions. However, in the unit cell of BaFe11.9Al0.1O19 below the Curie temperature, there is also a distortion to a lowersymmetry phase accompanied by the shift off-center of the small Fe cation [35]. We were the first who investigated and analyzed temperature dependences of BaFe11.9Al0.1O19 behaviour Fe-O bond lengths and Fe-O-Fe angles versus temperature in the range 4-730 K [35]. For the studied sample no abrupt changes of the internal structural parameters over the entire temperature range from 4 K up to 750 K were found. All the bond lengths decrease with temperature decreasing. The greatest changes in the bond lengths are detected for the Fe3 cation 4

relative to the O2 cation, for the Fe4 cation relative to the O5 cation and for the Fe5 cation relative to the O2 cation Fig. 3 [35]. Almost all the bond angles decrease with temperature decreasing except for the Fe3-O4-Fe5. The greatest changes in the bond angles are detected for the Fe4-O3Fe4, Fe5-O1-Fe5 (they decrease) and Fe3-O4-Fe5 (it increases) as temperature decreases. In our previous papers [35, 39] we focused on the behavior of the internal structural parameters as a function of temperature since it helped us to understand the nature of magnetic and dielectric properties. We demonstrated that in BaFe11.9Al0.1O19 in the wide temperature range there is anomaly microstructural transition on atomic level – non-centrosymmetric shifts of iron cation in 12k. So it is the evidence of non-centrosymmetric SG formation. In our previous papers [35-42] we also described crystal structure of substituted M-type hexaferrites by SG #194 - P63/mmc. Recently we concluded that it was our mistake. Our mistake and the mistake most of the other researchers who dealt with dual ferroic properties in M-type of hexaferrites and described crystal structure by P63/mmc. Because theoretically there can’t be polarization in centrosymmetrical SG. SG P63/mmc does not allow the formation of non-zero dipole electrical moment. Our careful analysis of features of hexagonal structure leads us to the suggestion that this sample (its crystal structure) may be described also by “non-cenrosymmetrical” SG #186 (P63mc) – Fig. 2 b. We refined NPD spectra of BaFe11.9Al0.1O19 in approximation for SG #186 (“noncentrosymmetrical” polar phase). Atomic coordinates and main parameters: Rwp (weighted profile R-value), Rexp (expected R-value), RB (Bragg R-factor), RMag (magnetic R-factor) and χ2 (goodnessof-fit quality factor) parameters obtained after refinement in approximation for SG #186 are presented in Table 2. It is clear that goodness-of-fit quality factor (χ2) demonstrates good agreement between experimental data and theoretical model for SG #194 and for #186 too. We think that there are at least 2 main phases in substituted M-type hexaferrites: SG: #194 and SG: #186 which could describe the features of crystal structure in Al-substituted sample. The dependences of lattice parameters versus temperature for both approximation (#194 and #186) is shown at Fig. 4. The volume of unit cell of the Al-doped barium hexaferrite is lower than pure BaFe12O19 composition [35]. When the temperature decreases the lattice parameters and the volume of the unit cell decrease. It is due to the decreasing of the thermal energy of the random motion of ions. But it is an interesting fact that goodness-of-fit quality factor (χ2) at low temperatures (4.2150 K) is better for SG #186 approximation. And at high temperature (630-730 K) χ2 is better for SG #194 approximation (compare tab.1 and tab.2). The values of a, c parameters and V in these regions are approximately equal (Fig. 4). And in the region 150 K-630 K the unit cell parameters are rather different and χ2 is not so good in comparison with another temperature regions. We think that it could be explained by two different phases in BaFe11.9Al0.1O19: polar phase – SG #186 (at low temperatures) and non-polar phase – SG #194 (at high temperatures) and mutual phase transitions on atomic level (SG #186 and SG #194 very similar but in SG#186 can be noncentrocymmetric shifts which lead to non-zero dipole moment). We concluded that in the region 150 K-630 K there can be coexistence of two different phases at the same time (phase separation due to frustration of magnetic structure). But most researchers who investigated M-type hexaferites do not think about this because for more than 60 years these kind samples described only SG #194. Fig. 5 shows the grain topography obtained by scanning electron microscopy (pictures of SEM in [35]) for the Al-substituted BaFe11.9Al0.1O19 hexaferrite. As it can be observed, the grains combine to form a mosaic structure spanning the entire ceramic. The grain size variation interval is 0.195  2.071 µm (Tab. 3). For the BaFe11.9Al0.1O19 hexaferrite 62.1 % of the grains have a size variation from 0.710 µm to 0.860 µm. The grains with size smaller than 0.120 µm and larger than 5

1.200 µm have been not detected more than 4 % (Fig. 5). The precise value of the average grain size for this sample from the quantitative stereologic analysis is  D   0.893 μm. 3.2 Magnetic properties According to magnetic investigations [35] the Tc ferrimagnet-paramagnet phase transition temperature for the BaFe11.9Al0.1O19 is 705 K. Whereas for the un-doped BaFe12O19 the Curie temperature is equal to 740 K [39]. For the un-doped BaFe12O19 samples produced by coprecipitation, herein, the Tc value obtained was 725 K at an Ms equalling the zero point. Authors of [43] obtained two different Tc measurements, one where the samples were sintered within a 0.25 T field, causing the resulting compound to become aligned that found a Tc of 452 0C or 726 K and without a field during sintering as with [44] and the present samples, Wang obtained a Tc of 410 0C or 683 K. The magnetic moments are thought to be partially aligned within magnetic domains in the samples. In the present case, the samples were not sintered in the presence of a magnetic field. It is therefore suggested that their original samples were, in some way, contaminated as evidenced by a lower value of Tc observed on their standard material samples. Tc can also be expressed in terms of the number of Fe3+–O2-–Fe3+ indirect superexchange interactions. This is in agreement with a similar explanation provided by [45]. The presense of the diamagnetic Al3+ cations in the solid solution of barium hexaferrite leads to reducing of the number of neighbors of magnetic iron cations and so that the magnetic order is destroyed at lower temperatures [46]. So at 4 K in field of 1 T the specific magnetization is 69 emu/g. This is lower than for the undoped sample. The reduction of the magnetization was caused by the incomplete coordination of the atoms on the particle surface leading to a noncollinear spin configuration, which causes the formation of a surface spin canting. Moreover, the reduction was made because of the thermal fluctuation of magnetic moments, which significantly diminishes the total magnetic moment for a given magnetic field. The specific magnetization continuously decreases from 4 K up to 730 K. This peculiar shape of the M(T) curves in hexaferrites is due to the temperature dependence of the magnetic moments of iron cations on the 12k site – Fe5. Such temperature variations of the hyperfine fields of the 12k sites and the subsequent increase of curvature of M(T) with x have already been demonstrated for Sr1-xLaxFe12-xCoxO19 ferrites [47-49]. This broad transition to the paramagnetic state resembles a second-order phase transition. From this measurement the absence of the temperature hysteresis of the specific magnetization is observed which is also characteristic of the second-order phase transition. The continuous decrease of the specific magnetization may be understood in term of destruction of the intersublattice exchange interactions initially and of the intrasublattice Fe3+-O2Fe3+ indirect superexchange interactions further. It is known [50] that for the perovskite-like compounds with general ABX3 formula, it is a straightforward result from tight-binding approximation that τ intensity of indirect superexchange interactions depends on both the B-X-B bond angles and B-X bond lengths through the overlap integrals between the 3d-orbitals of the metal B ion and the 2p-orbitals of the X anion. The following empirical formula has been used to describe this double dependences : 1 cos (  B  X  B 2  3.5 BX

(2)

< B-X > - average bond length, < B-X-B > - average bond angle. Based on this modeling approach and data of [35] it can be concluded that the main contribution to the weakening of the magnetic ordering at low temperatures follows from the destruction of the intersublattice exchange interactions between Fe3, Fe4, Fe5 sublattices. Fe4 and Fe5 cations may be responsible for the polarization origin. 6

The saturation of specific magnetization values decrease with temperature increasing as observed from the graph of the field dependence isotherms of the specific magnetization (Fig. 6). The sample goes into saturation in external magnetic fields up to 0.2 T at almost all temperatures below Tc [51]. From this macroscopic measurement the consolidated magnetic moment at 4 K in field of 2 T is 14.5 µB per formula unit or 1.2 µB per nominal Fe3+ cation. Taking into account the magnetic structure of M-type hexaferrite according to Gorter’s model [52] with iron up-spins on 12k, 2a and 2b sites and down-spins on 4fIV and 4fVI sites, a moment per formula unit of 20 µB is expected for the BaFe12O19 in perfect collinear case. Assuming the Al3+ cations to be located at up-spin 12k-Fe5 positions a moment of 19.5 µB per formula unit is expected for the BaFe11.9Al0.1O19. The measured saturation magnetization is smaller compared to calculated one. Some reduction of the magnetization might be caused by above mentioned reasons. The nature of the ferrimagnet–paramagnet phase transition may be determined using the magnetic criterion proposed by Banerjee [53]. To refine the features of the behavior of the magnetic system under study at the magnetic transition, we measured the field isotherms of specific magnetization and constructed Arrott’s plots [54] (Fig. 6 insert). In the model approximation of the molecular field theory, for the ground state of the system, the square of magnetization M2 is proportional to the ratio B/M. This relation strictly holds for high magnetic fields close to the saturation fields. For low fields, a deviation from the linear behavior is observed. A negative value corresponding to the point of intersection of the linearly extrapolated Arrott’s isotherms points to the ordered state of the system, whereas a positive value indicates the disordered state. According to the Banerjee criterion, a positive value of the slope of the tangent to the Arrott’s isotherms at any point in the ordered state determines a second-order magnetic phase transition, whereas a change from the positive slope of the tangent to a negative one means a first-order phase transition. A positive value of the slope of the tangent corresponds to a rising Arrott’s isotherm or, what is the same, to a positive value of the derivative d(M2)/d(B/M) [55]. Thus, we conclude that the magnetic phase transition that occurs for the BaFe11.9Al0.1O19 at ~ 705 K is a second-order thermodynamic phase transition from the ferrimagnetic to paramagnetic state. This conclusion is also confirmed by the absence of temperature hysteresis for the M(T). 3.3 Magnetic structure In hexaferrites the magnetic Fe3 + ions are located in positions which have octahedral (Fe1-2a, Fe4-4fVI and Fe5-12k), tetrahedral (Fe3-4fIV) and bipyramidal (Fe2-2b) oxygen environment. As a result of partial replacement of iron ions by diamagnetic aluminum ions which are distributed statistically equivalent for all positions of the magnetic lattice it can be expected to change in the values of the magnetic moments in corresponding positions. In addition, according to [56], the magnetic moments of the above mentioned positions initially have slightly different values. The data were collected at different temperatures, including well below (< 705 K) the phase transition temperature for ferrimagnet-paramagnet. Tab. 4 shows the values of the temperature dependence of the magnetic moments of Fe3+ cations in different crystallographic positions: 2a, 2b, 4fIV, 4fVI and 12k. It is interesting that the data of the NPD spectra refinement for SG #186 and SG #194 approximations give us different values of magnetic moments of Fe3+ cations (compare Tab. 4 and Tab. 5). The absence of the additional magnetic peaks which appear at temperatures below the paramagnet-ferrimagnet phase transition, allows to determine the wave vector of the ferromagnetic structure as k = [0,0,0]. The magnetic structure in all the temperature range fully satisfies the model proposed by Gorter [52], according to which, all the magnetic moments of the Fe3+ cations are oriented along the easy axis which coincides with the hexagonal c axis.

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As the substitution of iron cations by aluminum cations the exchange interactions between the magnetic positions and sublattices are broken which leads to a decrease in the value of their magnetic moments. A slight change in the magnetic moment appears to reduce the magnitude of the sublattice magnetic moments of the formed iron cations located at positions 2a and 2b may be due to inhomogeneities in the distribution of aluminum ions on crystallographic positions in the preparation of the samples. It can be assumed that at a low concentration of aluminum ions in the compound it should be distributed statistically throughout nonequivalent positions magnetic hexagonal ferrite lattice. However in our case to reduce the discrepancy between the experimental data and performed calculations of the magnetic and crystal lattices for the Al-doped barium hexaferrites the aluminum ions in a greater degree was preferred to use the substitution of iron ions into the 2a, 2b and 12k positions, while 4fIV and 4fVI - to a lesser degree. Since the Al ions distribution is largely influenced by the method of sample preparation, the confirmation of the correctness of our judgments on their distribution in the corresponding crystallographic positions is planned to be found in the course of further research. Resulting or total magnetic moment per formula unit (for Tab. 4 and Tab. 5) for the substituted M-type barium hexaferrite at T temperature can be calculated according [57] to the formula: M total (T )  1[m2a (T )]  1[m2b (T )]  2[m4 fIV (T )]  2[m4 fVI (T )]  6[m12k (T )]

(3) If the magnetic moment of Fe3+ ion at 0 K is equal 5μB then magnetic moment of pure BaFe12O19 ferrite will be equal to 20μB per formula unit. Such low values are explained by influence of diamagnetic Al ions and thermal factor causing the disorientation of the magnetic moments in space due to the increase of the thermal fluctuations of the ions forming the crystal lattice [58]. 3.4 Dual ferroic properties and strong correlation Ferroelectric properties were characterized using polarization hysteresis and pulse polarization measurements. The electric field-induced polarization (P) is demonstrated in [35]. It shows the ferroelectric hysteresis loops of BaFe11.9Al0.1O19 at room temperature in the electrical field up 100 kV/m [35]. The maximum polarization (PMAX), remanent polarization (Pr) and the coercive electric field (EC) obtained from the ferroelectric hysteresis loop for BaFe11.9Al0.1O19 were approximately ~5.89 mC/m2, ~5.13 mC/m2 and ~86 kV/m respectively. The coexistence of electrical polarization and magnetization in single-phase samples is evidence of the dual ferroic (multiferroic) properties. Demonstration of multiferroic properties at room temperatures is the opportunity for practical application of such kind materials. Fig. 6 shows the field dependences of the specific magnetization at room temperature in external electrical field (60 kV/m, E⊥ B) and without electrical field. The spontaneous magnetization was 52.6 emu/g. At the application of an external electric field of 60 kV/m, the spontaneous magnetization increases approximately by 4% (to 54.7 emu/g). This effect can be explained by an increase in the degree of the polarization of local spins of Fe3+ at the addition of the energy of the electric field to the system. The spontaneous polarization depends on the resistivity of the samples affects the magnitude and accuracy of the measurement of the magnetoelectrical effect. The breakdown field limits the spontaneous polarization, as well as the magnetoelectrical effect in the unsaturated state. The magnetoelectrical effect should not increase at the saturation polarization. Since the magnetoelectrical effect obtained in this work almost coincides with that obtained in [31, 41], the relation between the spontaneous polarization and magnetoelectrical effect is most likely nonlinear. 8

The mechanism of the appearance of spontaneous polarization in the substituted hexaferrite BaFe11.9Al0.1O19 is illustrated in [35]. In this case, it is impossible to explain the appearance of the polarization by the formation of noncollinear magnetic structure. An explanation should be sought in charge ordering. A perfect centrosymmetric oxygen octahedron with a small iron cation at the center has zero polarization vector. The non-centrosymmetric distortion of this octahedron appearing at the displacement of the iron cation toward one of oxygen anions leads to the appearance of a nonzero dipole electric moment and, as a result, to spontaneous polarization. Fig. 8 demonstrates the temperature dependences of the spontaneous polarization – Ps (a), specific magnetization – (b) and magnetoelectrical coefficient – Kme (c) in the range 4.2-730 K. It has been seen that spontaneous polarization exists only in the range 4.2-400 K. Temperature increasing from 4.2 K to 400 K leads to decreasing Ps from 7.9 to 1.6 mC/m2 (Fig. 8 a). Formation non-zero dipole moment (polarization) is possible only in noncentrosymmetric phase (SG #186). So we can conclude that the features of the crystal structure and phase composition (on atomic level) in the temperature range 4.2-400 K can be described by noncentrosymmetric phase (SG #186) or coexistence of non-centrosymmetric phase (SG #186) and centrosymmetric phase (SG #194) in the sample (phase separation). We think that decreasing of P s when temperature increases can be explained by not only weakening of the non-centrosymmetric covalent bonds (in 12k-position) due to thermal fluctuations of ions. But changing SG #186/SG #194 ratio (SG #186 decreases and SG #194 increases) due to phase transition on atomic level. The specific magnetization continuously decreases from 4 K up to 730 K (Fig. 8b, it was in detail discussed above in the 3.2 section). Magnetoelectrical effect (Kme) was observed only at room temperature and at temperatures below the room. Temperature increasing from 4.2 K to 300 K lead to decreasing Kme from 8.1 % to 4 % (Fig. 8 c). The strong correlation between magnetic and electrical properties (magnetoelectrical effect) allows us to say that BaFe11.9Al0.1O19 is not only multiferroic (coexistence of specific magnetization and spontaneous polarization) but II-type multiferroic. Conclusions M-type BaFe11.9Al0.1O19 hexaferrite was successfully synthesized by solid state reactions. It has been performed precision investigations of crystal and magnetic structures of BaFe11.9Al0.1O19 powder by neutron diffraction in the temperature range 4.2 – 730 К. All researchers who deal with M-type hexaferrites as a rule described crystal structure by SG #194 (P63/mmc). This SG is centrosymmetric structure or “non-polar” phase. Our early paper [35] corresponded about coexistence of dual ferroic properties (magnetization and polarization) at room temperature in BaFe11.9Al0.1O19. Theoretically it can’t be in centrosymmetric SG non-zero dipole moment (electrical polarisation). In paper [35] we demonstrated that in BaFe11.9Al0.1O19 there is anomaly microstructural transition on atomic level – temperature induced non-centrosymmetric shifts of iron cation in 12k-position. So it is the evidence of transition to non-centrosymmetric SG. In this paper NPD data were successfully refined in approximation for both space groups (SG): centrosymmetric #194 (standard non-polar phase) and non-centrosymmetric #186 (polar phase). It has been shown that goodness-of-fit quality factor (χ2) demonstrates good agreement between experimental data and theoretical model for both SG #194 and for #186. Based on this we concluded that there are at least 2 main phases in substituted M-type hexaferrites: SG: #194 and SG: #186 which could describe the features of crystal structure in Al-substituted sample. It is an interesting fact that goodness-of-fit quality factor (χ2) at low temperatures (4.2-150 K) is better for SG #186 approximation. And at high temperature (630-730 K) χ2 is better for SG #194 approximation. Magnetic properties of the BaFe11.9Al0.1O19 were investigated by VSM in the wide temperature range. Ferrimagnet-paramagnet phase transition temperature for the BaFe11.9Al0.1O19 is 705 K. The nature of the ferrimagnet– paramagnet phase transition was determined using the magnetic criterion proposed by Banerjee.

9

The specific magnetization continuously decreases from 4 K up to 730 K. Consolidated magnetic moment at 4 K in field of 2 T is 14.5 µB per formula unit or 1.2 µB per nominal Fe3+ cation. Magnetoelectrical effect was estimated from field dependences of the specific magnetization at room temperature in external electrical field (60 kV/m, E⊥B) and without electrical field. The spontaneous magnetization without magnetic field was 52.6 emu/g. At the application of an external electric field of 60 kV/m the spontaneous magnetization increases approximately by 4% (to 54.7 emu/g). Temperature increasing from 4.2 K to 400 K leads to decreasing Ps from 7.9 to 1.6 mC/m2. We think that decreasing of Ps when temperature increases can be explained by not only weakening of the non-centrosymmetric covalent bonds (in 12k-position) due to thermal fluctuations of ions. But also by changing SG #186/SG #194 ratio (SG #186 decreases and SG #194 increases) due to phase transition on atomic level. Magnetoelectrical effect (Kme) was observed only at room temperature and at temperatures below the room. Temperature increasing from 4.2 K to 300 K leads to decreasing Kme from 8.1 % to 4 % The strong correlation between magnetic and electrical properties (magnetoelectrical effect) allows us to say that BaFe11.9Al0.1O19 is not only multiferroic (coexistence of specific magnetization and spontaneous polarization) but II-type multiferroic. Coexistence of dual ferroic properties and magnetoelectrical effect at room temperatures opens opportunity for creation and development new devices for the modern branch of microelectronics - magnetoelectronics and sensors. We think that our opinion about possibility of describing M-type hexaferrites by noncentrocymmetric polar phase (SG: #186) and about possibility of the coexistence 2 phases SG: #186 and SG: #194 in the sample (phase separation and mutual temperature induced phase transition) will be interesting for other researchers. We hope that it will be helpful for explanation of the reason of spontaneous polarization formation in systems which were described by centrosymmetric SG for a long time. Coexistence of dual ferroic properties and magnetoelectrical effect at room temperatures opens opportunity for creation and development new devices for the modern branch of microelectronics - magnetoelectronics and sensors. We think that our opinion about possibility of describing M-type hexaferrites by noncentrocymmetric polar phase (SG: #186) and about possibility of the coexistence 2 phases SG: #186 and SG: #194 in the sample (phase separation and mutual temperature induced phase transition) will be interesting for other researchers. We hope that it will be helpful for explanation the reason of spontaneous polarization formation in systems which were described by centrosymmetric SG for a long time. Acknowledgement The work was carried out with financial support in part from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST«MISiS»(№ К4-2015-040 and № K3-2016-019) and in part by the Belarusian Republican Foundation for Fundamental Research (Grant No. F15D-003) and Joint Institute for Nuclear Research (Grant No. 04-4-1121-2015/2017). L. Panina acknowledges support under the Russian Federation State contract for organizing a scientific work.

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39. S.V. Trukhanov, A.V.Trukhanov, V.A.Turchenko, V.G.Kostishin, L.V.Panina, I.S. Kazakevich, A.M.Balagurov, J. Magn. Magn. Mater. 417 (2016) 130 40. V.A. Turchenko, A.V. Trukhanov, I.A. Bobrikov, S.V. Trukhanov, A.M. Balagurov, Crystallogr. Rep. 60 (2015) 629. 41. V.G. Kostishyn,L.V.Panina, АV. Timofeev,L.V.Kozhitov,A.N.Kovalev, A. K. Zyuzin,J.Magn.Magn.Mater.400(2016)327. 42. A.V. Trukhanov, L.V. Panina, S.V. Trukhanov, V.A. Turchenko, M. Salem, Chin. Phys. B 25 (2016) 016102. 43. J. Wang, Q. Chen, S. Che, J. Magn. Magn. Mater. 280 (2004) 281. 44. S. Ram, H. Krishnan, K.N. Rai, K.A. Narayan, Jpn. J. Appl. Phys. 28 (4) (1989) 604. 45. M.A. Gilleo, Phys. Rev. 109 (1958) 777. 46. S.V. Trukhanov, JETP 100 (2005) 95. 47. M.Le Breton, J. Teillet, G. Wiesinger, A. Morel, F. Kools, P. Tenaud, IEEE Trans. Magn. 38 (2002) 2952. 48. C. Sauer, U. Köbler, W. Zinn, H. Stäblein, J. Phys. Chem. Solids 39 (1978) 1197. 49. D. Seifert, J. Töpfer, F. Langenhorst, J.-M. Le Breton, H. Chiron, L. Lechevallier, J. Magn. Magn. Mater. 321 (2009) 4045. 50. P.G. Radaelli, G. Iannone, M. Marezio, H.Y. Hwang, S.-W. Cheong, J.D. Jorgensen, 51. D.N. Argyriou, Phys. Rev. B 56 (1997) 8265. 52. S.V. Trukhanov, A.V. Trukhanov, C.E. Botez, A.H. Adair, H. Szymczak, R. Szymczak, J. Phys.: Condens. Matter. 19 (2007) 266214. 53. E.W. Gorter, Proc. IEEE Suppl. 104B, 225 (1957). 54. S.K. Banerjee, Phys. Lett. 12 (1964) 16. 55. A. Arrott, J.E. Noakes, Phys. Rev. Lett. 19 (1967) 786. 56. S.V. Trukhanov, A.V. Trukhanov, A.N. Vasil’ev, A. Maignan, H. Szymczak, JETP Letters 85 (2007) 507. 57. X. Batlle, X. Obradors, J. Rodriguez-Carvajal, M. Pernet, M.V. Cabanasans, M. Vallet, J. Appl. Phys. 70 (1991) 1614. 58. J. Smit, H.P.J. Wijn Ferrites, Cleaver, Hume Press Ltd. (1959), p. 142. FIGURE CAPTIONS Fig. 1. Powder neutron diffraction pattern for the BaFe11.9Al0.1O19 sample measured by HRFD at 4.2 K (a) and 730 K (b) and processed by Rietveld method. It shows the experimental points (crosses), calculated function (curve), difference curve (lower curve) normalized to the statistical error and diffraction peak positions (vertical bars) for the atomic (A) and magnetic (B) structure of the BaFe11.9Al0.1O19. For 4 K (a) pattern the reflections from the copper sample holder are marked. Fig. 2. Features of the #194 or P63/mmc (a) and #186 or P63mc (b) space groups (with possible symmetry operators) Fig. 3. Schematic structure of the BaFe11.9Al0.1O19 and different anion coordination in M-type hexaferrites Fig. 4. The temperature dependences of the refined (a and c) lattice parameters and V unit cell volume for the BaFe11.9Al0.1O19 sample within space group P63mc (№186) and P63/mmc (№194) approximation. Fig. 5.The grain topography at 300 K for the BaFe11.9Al0.1O19 sample Fig. 6. The field dependences of the atomic magnetic moment for the BaFe11.9Al0.1O19 sample at different temperatures. Insert demonstrates the Arrott’s plots - the dependences of the M2 specific magnetization square on the B/M ratio for different temperatures near the Curie point for the BaFe11.9Al0.1O19 sample. 12

Fig. 7. Field dependence of the specific magnetization at room temperature (300 K) in an external electric field of 60 kV/m and without a field for the BaFe11.9Al0.1O19 hexaferrite (magnetoelectrical effect is evidence of strong correlation between dual ferroic properties). The inset shows this dependence on a magnified scale. Fig. 8. Temperature dependences of the spontaneous polarization – Ps (a), specific magnetization –  (b) and magnetoelectrical coefficient – Kme (c)

Table 1. The atomic coordinates and the standard relevance factors of calculation and experiment for the BaFe11.9Al0.1O19 sample obtained by powder neutron diffraction on HRFD and calculated by Rietveld method within space group P63/mmc (№194) at different temperatures. T, K atoms Fe3 (4fIV) z Fe4 (4fVI) z Fe5 (12k) x z O1 (4e) z O2 (4f) z O3 (6h) x O4 (12k) x z O5 (12k) x z Rwp, % Rexp, % RB, % RMag, % χ2

4.2

150

300

630

730

0.0275(2)

0.0283(3)

0.0276(2)

0.0272(2)

0.0277(1)

0.1896(2)

0.1903(2)

0.1894(1)

0.1896(1)

0.1889(1)

0.1669(7) -0.1086(1)

0.1676(8) -0.1086

0.1673(5) -0.1081(6)

0.1677(4) -0.1082(1)

0.1681(4) -0.1084(1)

0.1497(3)

0.1499(3)

0.1497(2)

0.1500(2)

0.1505(2)

-0.0557(3)

-0.0557(4)

-0.0552(2)

-0.0558(2)

-0.0551(2)

0.1882(12)

0.1875(15)

0.1863(9)

0.1843(10)

0.1831(8)

0.1545(9) 0.0522(2)

0.1546(11) 0.0522(2)

0.1548(7) 0.0515(1)

0.1545(7) 0.0516(1)

0.1541(6) 0.0518(1)

0.5077(15) 0.1493(1) 11.5 8.88 6.16 6.04 1.69

0.5068(19) 0.1489(1) 14.5 11.00 10.2 15.4 1.79

0.5040(11) 0.1490(1) 11.4 8.08 5.17 9.50 1.98

0.5044(9) 0.1486(1) 11.1 8.78 9.34 13.5 1.29

0.5051(8) 0.1480(1) 10.7 7.88 8.52 -1.86

Table 2. The atomic coordinates and the standard relevance factors of calculation and experiment for the BaFe11.9Al0.1O19 sample obtained by powder neutron diffraction on HRFD and calculated by Rietveld method within space group P63mc (№186) at different temperatures. T, K atoms Ba (2b) z Fe1 (2a) z Fe2 (2a) z Fe3 (2b) z Fe33 (2b)

4.2

150

300

630

730

0.25

0.25

0.25

0.25

0.25

0

0

0

0

0

0.25

0.25

0.25

0.25

0.25

0.0312(9)

0.0305(9)

0.0275(7)

0.0251(5)

0.0241(4)

13

z Fe4 (2b) z Fe44 (2b) z Fe5 (6c) x z Fe55 (6c) x z O1 (2a) z O11 (2a) z O2 (2b) z O22 (2b) z O3 (6c) x z O4 (6c) x z O44 (6c) x z O5 (6c) x z O55 (6c) x z Rwp, % Rexp, % RB, % RMag, % χ2

0.4734(9)

0.4717(10)

0.4710(7)

0.4693(5)

0.4680(4)

0.1889(4)

0.1949(7)

0.1888(2)

0.1899(2)

0.1889(5)

0.3108(0)

0.3141(7)

0.3108(0)

0.3108(0)

0.3123(5)

0.1658(13) -0.1074(0)

0.1713(0) -0.1079(2)

0.1682(7) -0.1074(0)

0.1671(8) -0.1074(0)

0.1663(8) -0.1084(3)

0.1680(0) 0.6097(2)

0.1673(15) 0.6084(0)

0.1680(0) 0.6088(1)

0.1680(0) 0.6085(1)

0.1680(0) 0.6085(3)

0.1522(6)

0.1552(6)

0.1527(3)

0.1517(4)

0.1501(10)

0.3521(0)

0.3544(0)

0.3531(0)

0.3531(0)

0.3489(10)

0.9413(5)

0.9454(0)

0.9406(0)

0.9406(0)

0.9404(4)

0.5505(0)

0.5554(7)

0.5505( 0)

0.5505(0)

0.5465(0)

0.1878(12) 0.25

0.1868(15) 0.25

0.1853(8) 0.25

0.1834(9) 0.25

0.1841(8) 0.25

0.1619(33) 0.0517(3)

0.1483(24) 0.0534(0)

0.1504(0) 0.0496(2)

0.1504(0) 0.0493(2)

0.1518(22) 0.0495(4)

0.1497(26) 0.4452(0)

0.1657(35) 0.4493(4)

0.1614 (14) 0.4449(0)

0.1588(14) 0.4449(0)

0.1593(26) 0.4444(4)

0.5032(0) 0.1524(2)

0.5154(28) 0.1567(3)

0.5097(16) 0.1506(2)

0.5128(15) 0.1496(2)

0.5057(0) 0.1501(0)

0.5165(23) 0.3543(0) 11.4 8.87 6.24 6.43 1.56

0.4975(36) 0.3611(0) 13.6 10.20 7.64 12.2 1.67

0.4938(16) 0.3530(0) 10.5 8.28 5.32 6.71 1.60

0.4960(17) 0.3530(0) 10.8 9.39 7.29 11.9 1.32

0.5006(0) 0.3538(2) 11.4 8.23 8.96 -1.91

Table 3. The results of the quantitative stereologic analysis of the reflection electron microscopy images of the BaFe11.9Al0.1O19 sample at 300 K. Parameter Value Maximal size, (µm) 2.071 Minimal size, (µm) 0.195 Porosity, (%) 4.15 Simple average, (µm) 0.631 Geometric average, (µm) 0.602 Harmonic average, (µm) 0.685 Sampling variance 0.014 Sampling mean-square variance 0.137 Mean-square variance from average 0.014 Aggregate dispersion 0.013 Aggregate mean-square variance 0.141 Relative error 0.049 Skewness 1.052 14

Kurtosis Magnitude sum Magnitude quantity

3.732 39.12 65

Table 4. The magnetic moment (μB) per iron ion in different positions obtained by powder neutron diffraction on HRFD and calculated by Rietveld method at different temperatures within SG #194. T, K Fe1 (2a) Fe2 (2b) Fe3 (4fIV) Fe4(4fVI) Fe5 (12k) Mtotal, μB/f.u.

4.2 4.74 4.24(16) 3.87(10) 4.277 4.30(4) 18.45

150 4.60(31) 4.35(32) 3.57(13) 3.88(16) 3.69(6) 16.17

300 3.42(8) 3.42(8) 3.90(9) 3.17(9) 3.59 14.25

630 2.37(12) 2.37(12) 2.72(15) 2.20(14) 2.09 7.39

730 0 0 0 0 0 0

Table 5. The magnetic moment (μB) per iron ion in different positions obtained by powder neutron diffraction on HRFD and calculated by Rietveld method at different temperatures within SG #186. T, K Fe1 (2a) Fe2 (2a) Fe3 (2b) Fe33 (2b) Fe4 (2b) Fe44 (2b) Fe5 (6c) Fe55 (6c) Mtotal, μB/f.u.

4.2 4.83 4.83 4.10(6) 4.10(6) 4.17(6) 4.17(6) 4.26(4) 4.26(4) 18.83

150 4.38(11) 4.38(11) 3.98(13) 3.98(13) 3.41(16) 3.41(16) 3.76(6) 3.76(6) 16.55

300 3.82(8) 3.82(8) 4.13(10) 4.13(10) 4.08(11) 4.08(11) 3.42(4) 3.42(4) 11.72

630 3.06(19) 2.82(0) 2.95(15) 2.95(15) 2.65(15) 2.65(15) 1.98(0) 1.87(0) 6.26

730 0 0 0 0 0 0 0 0 0

15

Fig. 1

16

Fig. 2

17

Fig. 3

18

Fig. 4

19

Fig. 5

20

Fig. 6

21

Fig. 7

22

Fig. 8

23